A 4G communication system capable of transmitting large-size data at high speed uses an OFDM (Orthogonal Frequency Division Multiplex) or OFDMA (Orthogonal Frequency Division Multiplex Access) method. The OFDM or OFDMA method divides a bandwidth into plural subcarriers, and transmits the subcarriers. That is, the OFDM or OFDMA method converts data rows provided in series into ‘N’ data columns provided in parallel; and then transmits the converted data columns while being carried on the respective subcarriers, to thereby improve data rate.
Also, an MIMO (Multi-Input Multi-Output) method is used for improving efficiency of data transmission in a communication system using the OFDM or OFDMA method, wherein the MIMO method transmits different data by the use of respective antennas provided in a base station. The MIMO method enables to obtain a diversity gain in the communication system, and also to improve data transmission rate.
By the use of MIMO method, data or signals may be received and transmitted between the base station and plural mobile stations. In case of this multi-user (MU) MIMO method, ‘M’ antennas are disposed in the base station; and ‘N’ antennas are disposed in the plural mobile stations. When the signals are received and transmitted by the use of disposed antennas, the data transmission rate is improved.
In order to provide MU multi-antenna service via the multi-antenna in the MU-MIMO method, the base station needs channel state information (CSI) of the plural mobile stations.
In the communication system of TDD (Time Division Duplex), since an uplink channel is symmetric to a downlink channel in channel properties, the channel state information can be easily obtained by the use of sounding symbols. Meanwhile, in the communication system of FDD (Frequency Division Duplex), since an uplink channel is not symmetric to a downlink channel in channel properties, feedback information including the channel state information (CSI) should be directly transmitted from the plural mobile stations.
A method for transmitting a frame in the communication system of FDD using the MU-MIMO method will be described as follows.
First, when the respective mobile stations feedback the channel state information generated by the use of common pilot transmitted from the base station to the base station, the base station groups some of the plural mobile stations, and calculates a precoding matrix for the grouped mobile stations by the use of channel state information about the grouped mobile stations. After precoding the data to be transmitted to the grouped mobile stations by using the precoding matrix, the precoded data together with a dedicated pilot precoded for detection of the precoded data and a common pilot for channel estimation is transmitted to the respective mobile stations.
The precoding is among the methods for improving reliability of the data transmission in the wireless communication system. That is, the data to be transmitted is precoded by the use of precoding matrix generated based on information for fading and interference occurring on the channel for the data transmission, and then the precoded data is transmitted, to thereby improve reliability of the data transmission.
As mentioned above, in case of the general MU-MIMO system of FDD method, the common pilot for channel estimation and the dedicated pilot for data detection should be included in the downlink data. Thus, if the number of transmit-antennas is increased in the base station, there are too many pilots contained in the downlink frame, whereby the increase of overhead in the downlink frame may cause waste of radio resource.